The invention relates to an improved method of producing double metal cyanide (DMC) catalysts for the production of polyether polyols by polyaddition of alkylene oxides to starter compounds containing active hydrogen atoms.
Double metal cyanide (DMC) catalysts for the polyaddition of alkylene oxides to starter compounds containing active hydrogen atoms have been known for a long time (see, for example, U.S. Pat. Nos. 3,404,109, 3,829,505, 3,941,849 and 5,158,922). The use of said DMC catalysts for the production of polyether polyols effects, in particular, a reduction of the proportion of monofunctional polyethers having terminal double bonds, so-called monools, compared with the conventional production of polyether polyols by means of alkali catalysts, such as alkali hydroxides. The polyether polyols thus obtained can be processed to form high-quality polyurethanes (for example, elastomers, foams, coatings).
DMC catalysts are usually obtained by reacting an aqueous solution of a metal salt with the aqueous solution of a metal cyanide salt in the presence of an organic complex ligand, for example an ether. In a typical catalyst preparation, for example, aqueous solutions of zinc chloride (in excess) and potassium hexacyanocobaltate are mixed and dimethoxyethane (glyme) is then added to the dispersion formed. After filtering and washing the catalyst with aqueous glyme solution, an active catalyst of the general formula
Zn3[Co(CN)6]2xZnCl2yH2Ozglyme
is obtained (see, for example, EP-A 700 949).
The conventional method of producing DMC catalysts is to mix aqueous solutions of a metal salt and a metal cyanide salt in a stirred reactor in the presence of one or more organic complex ligands with the formation of a catalyst dispersion. In order to achieve high catalyst activity, vigorous stirring with high shear is in general necessary. A disadvantage of this is that, if large stirred reactors are used, a high energy expenditure is necessary because of low power densities and a severely non-uniform power density distribution is present. Normally, the power density of stirred reactors is about 104 W/m3. A further disadvantage is that foam formation occurs in the reactor during vigorous stirring, which results in a reduction of catalyst yield and activity.
U.S. Pat. No. 5,891,818 describes an improved method of producing DMC catalysts with increased catalyst yield and activity, and also reduced particle size, in which the DMC catalyst dispersion is produced in a stirred reactor, wherein some of the catalyst dispersion circulates and is sprayed into the reactor head space and the circulating flow is conveyed through a xe2x80x9chigh-shear in-line mixerxe2x80x9d. This is to be understood as meaning, for example, a high speed rotor-stator mixer. A disadvantage of this method is that the foam formation cannot be completely suppressed and that repeated circulation of the entire reactor content through the xe2x80x9chigh-shear in-line mixerxe2x80x9d is necessary in order to achieve uniform catalyst activity and particle size. Furthermore, the energy expenditure necessary for the stirring is very high and the combination of a stirred reactor with a circulation loop, a xe2x80x9chigh-shear in-line mixerxe2x80x9d and spray nozzles is expensive in terms of equipment.
It has now been found that DMC catalysts having a further increased activity, reduced particle size and narrower particle size distribution can be obtained if the DMC catalyst dispersion is produced using a mixing nozzle, preferably a jet disperser. Further advantages of this method compared with the method variant described in U.S. Pat. No. 5,891,818 are the lower energy expenditure during the shearing, the higher power density (approximately 1013 W/m3) compared with the use of a stirred tank, the problem-free extrapolation to a larger scale and the markedly simplified equipment construction as a result of the omission of the stirred reactor. Polyether polyols that are obtained by means of DMC catalysts that are produced by this method have, in addition, a reduced viscosity compared with polyether polyols that are obtained by means of DMC catalysts that are produced by the method described in U.S. Pat. No. 5,891,818.